cache: Rework cache API

The cache operations must be quick, optimized and possibly inlined. The
current API is clunky, functions are not inlined and passing parameters
around that are basically always known at compile time.

In this patch we rework the cache functions to allow us to get rid of
useless parameters and make inlining easier.

In particular this changeset is doing three things:

1. `CONFIG_HAS_ARCH_CACHE` is now `CONFIG_ARCH_CACHE` and
   `CONFIG_HAS_EXTERNAL_CACHE` is now `CONFIG_EXTERNAL_CACHE`

2. The cache API has been reworked.

3. Comments are added.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>